This invention generally relates to an improved camera and, particularly, to a perspective-controlled video camera for use with a system for measuring the diameter of silicon crystals grown by the Czochralski process to control an apparatus or method employing the Czochralski process.
Crystal pulling machines employing the Czochralski process produce the substantial majority of monocrystalline silicon used to make silicon wafers for the microelectronics industry. Briefly described, the Czochralski process involves melting chunks of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace to form a silicon melt. The lower end of a pull wire hanging from a crystal lifting mechanism suspends a relatively small seed crystal above the crucible. The crystal lifting mechanism lowers the seed crystal into contact with the molten silicon in the crucible. When the seed begins to melt, the mechanism slowly withdraws it from the silicon melt. As the seed is withdrawn, it grows drawing silicon from the melt. During the growth process, the crucible rotates in one direction while the crystal lifting mechanism, wire, seed, and crystal rotate in an opposite direction.
As crystal growth is initiated, the thermal shock of contacting the seed with the melt may cause dislocations in the crystal. Unless eliminated in the neck region between the seed crystal and the main body of the crystal, the dislocations propagate throughout the growing crystal and multiply. The known methods of eliminating dislocations within silicon single crystal involve growing a neck having a small diameter at a relatively high crystal pull rate to completely eliminate dislocations before growing the body of the crystal. After dislocations are eliminated in the neck, its diameter is enlarged until the desired diameter of the main crystal body is reached. When the neck, which is the weakest part of the crystal, has too small of a diameter, it can fracture during crystal growth, causing the body of the crystal to drop into the crucible. The impact of the crystal ingot and splashing molten silicon can cause damage to the crystal growing apparatus as well as present a serious safety hazard.
As is known in the art, the Czochralski process is controlled, in part, as a function of the diameter of the crystal being grown. Thus, for both control and safety reasons, an accurate and reliable system for measuring crystal diameter, including neck diameter, is needed.
Several technologies are known for providing crystal diameter measurements, including methods of measuring the width of the bright ring. The bright ring is a characteristic of the reflection of the crucible wall in the meniscus which is formed at the solid-liquid interface. Conventional bright ring and meniscus sensors employ optical pyrometers, photocells, rotating mirrors with photocells, light sources with photocells, line-scan cameras, and two-dimensional array cameras. U.S. Pat. Nos. 3,740,563, 5,138,179 and 5,240,684, the entire disclosures of which are incorporated herein by reference, disclose methods and apparatus for determining the diameter of a crystal during the crystal growth process.
Presently available apparatus for automatically measuring crystal width, however, are often not sufficiently accurate or reliable for use during the different phases of crystal growth or for large diameter crystals in which the true maximum of the bright ring may be obscured from view by the solid body of the crystal itself. In an effort to correct this problem, apparatus for measuring crystal width attempt to measure the meniscus at a chord or at a single point along the meniscus. However, such apparatus require precise mechanical positioning of the scanning device and are highly sensitive to fluctuations in melt level. Further, conventional measuring apparatus require frequent calibration by the operator of the crystal growing apparatus to ensure that the diameter remains within specification.
Crystal measuring apparatus typically include a camera, such as a monochrome charge coupled device (CCD) camera, mounted in a viewport of the crystal growing chamber at an angle with respect to the axis of the growing crystal. The camera generates a video image of the crystal including an image of the meniscus at the interface between the silicon melt and the crystal. A disadvantage of such apparatus is that the position of the camera causes perspective distortion so that the image of the meniscus appears elliptical rather than circular. Although mathematical transformations are available for extracting a circular shape from a distorted elliptical shape to compensate for perspective distortion, such transformations are complicated and delay the performance of a vision system processing the image of the meniscus.
Commonly assigned application Ser. No. 08/459,765, filed Jun. 2, 1995, the entire disclosure of which is incorporated herein by reference, provides improvements in vision systems for use in measuring crystal diameter during the growing process. Although the system and method of application Ser. No. 08/459,765 provide improved crystal diameter measurements, there is still a need for a system that compensates for perspective distortion without the use of complicated transformations and additional processing steps by the vision system.
Telecentric lenses solve a related problem in which the field of view contains three-dimensional objects that are off-axis to the lens. However, the telecentric lens is only effective when the field of view is less than or only slightly greater than the diameter of the objective lens. As such, camera tilting or panning is still required when measuring objects that are outside the field of view.
For these reasons, conventional apparatus fail to provide a sufficiently accurate and reliable system of automatically determining crystal diameter for controlling the crystal growth process that compensates for perspective distortion.